The present invention relates in general to the field of audio systems having signal detection devices for detecting audio signals and more particularly, to a peak-to-peak signal detector for detecting dual channel L+R stereo audio signals.
The broadcasting of multichannel or stereo sound programs for television in the United States is in accordance with the system adopted by the Broadcast Systems Television Committee (BTSC) of the Electronic Industries Association (EIA). This television multichannel sound system provides for the transmission of the sum of the left and right stereo audio information (L+R) in a main audio channel in the spectrum space of the television signal formerly occupied by the monophonic audio signal (mono). This was done so that new stereo signals would be compatible with existing monophonic television receiver. Television multichannel sound systems also provide for the transmission of the difference of the left and right stereo audio information (Lxe2x88x92R), modulated onto a subcarrier.
The large multichannel sound systems employed in current consumer electronic audio units such as television receivers in combination with loud sound effects recorded on media including high-fidelity VHS video tapes and laser disks contribute to audio signals having wide dynamic range characteristics. Such system dynamic range capability is often an attractive feature in home theater audio/video systems currently on the market and generally contributes to the overall quality and enjoyment of the listener. However, while this may be true in general, sounds consisting of large amplitude transient signals can be disturbing and cause excessive noise. This may be most undesirable in certain instances, such as when children are sleeping. Not surprisingly, under these circumstances, it is desirable to include an automatic dynamic volume control compression system to limit the wide dynamic range performance over a range of audio signals. Accordingly, one must fit accurately detect and track such positive or negative amplitude audio signals in order to perform such dynamic volume control processes.
A peak signal detector may be used to detect a peak amplitude associated with an audio signal and utilized to provide a detector voltage which may then be used to control a volume setting so as to limit the loudness indicative of a high amplitude transient signal. However, audio signals are often asymmetrical. Therefore, examination and detection of only one of the portions of an audio (AC) signal may result in detection of a peak that may be too small (or too large) and thus, not a correct representation of the transient signal amplitude to be detected. That is, if a positive peak detector is used to detect an ac audio signal, then detection of only positive transient signals would occur. For negative excursion transient signals, the positive peak detector would be unable to detect these negative transients, and hence allow the loud signal to be received and companded by the television receiver unit, resulting in an undesirably loud audio signal perceived by the listener. Use of precision rectifiers and amplifiers to detect signal amplitudes of both polarities may be employed to detect both positive and negative going transient ac signals. However, such rectifiers tend to be somewhat complex and require a relatively larger number of electronic components to implement the functions. This is an undesirable characteristic for a circuit which is to be included within a consumer electronic unit, such as a television receiver, in today""s highly competitive market. For this reason, television manufactures have been searching for a low cost and highly reliable alternative to accurately detect both positive and negative amplitude audio signals in order to perform dynamic volume control processing for limiting wide dynamic range audio.
In accordance with the present invention, a peak to peak detector circuit for use in an audio system comprises a first amplifier having an input terminal for receiving an L+R (AC)audio signal and an output terminal which is serially coupled to a resistor in series with a capacitor for generating a variable dc voltage having a time vs. amplitude relationship corresponding to the input L+R AC audio signal. A clamping diode having a anode electrode coupled to a reference potential and having an cathode electrode coupled to the capacitor operates to limit negative amplitude excursions associated with the variable dc voltage to a predetermined minimal value. A rectifying diode having a anode electrode coupled to the cathode electrode of the clamping diode, and an cathode electrode coupled to a second capacitor to charge the second capacitor responsive to the amplitude of raid variable de voltage operates to produce a dc output signal proportional to the peak to peak amplitude of said L+R audio signal. The detector circuit further comprises a clamping circuit responsive to the voltage value at the cathode electrode of the rectifying diode for clamping the dc output signal at a maximum value.